If the tank is established then he is probably getting enough food from existing algae. If its not, he has probably ate some, but snails have VERY small mouths so they can't eat an entire wafer normally. I'd break it into small pieces, like 1/4 or less, throw it in near him at night before bed, and see if any of that is touched in the morning.

That's odd that it hasn't touched it all. How often are your feeding the wafers? I would only drop one maybe once, twice tops a week. Also you'll want to provide a slice of a veggie such as cucumber or zucchini, kale, spinach. They need calcium to keep their shells good. Lots of people only leave them in their 8 hrs or less but I leave mine in there for up to 2 days with no problems.

I bought the NLS wafers and I break it up into quarters. I usually feed the rest to my goldfish and pleco and the one quarter to my snail twice a week. With snails... gosh they are easier to feed than fish. I put the wafer on the ground in the tank and set him near it. He usually munches it to death.

Spinach and cucumbers, here. Blanching does wonders. Just don't leave the left overs in for more then a day or so. They'll cloud up the water. Those wafers never worked in my house. Veggies are so much better.